The Democratic Party of Japan, the country's ruling party, picked Noda as its new leader on Monday. He served as finance minister in Kan's cabinet.

In his first speech as party leader, Noda called for party unity to tackle Japan's massive problems.

"Running Japan's government is like pushing a giant snowball up a snowy, slippery hill," he said Sunday. "In times like this, we can't say, 'I don't like this person,' or 'I don't like that person.' The snowball will slide down."

On Friday, Prime Minister Kan announced that he would resign. His approval rating had tumbled following the devastating March earthquake and tsunami that triggered the worst nuclear crisis since Chernobyl as reactor cores overheated and spewed radioactive material into surrounding areas.

An observer of Japan's revolving door of prime ministers said the country's political problems are weighing down one of the world's largest economies.

Japanese politicians lack spine and public support, said Keith Henry of the Tokyo-based Asia Strategy, a government policy consulting firm.

"They've got to turn the ship around 180 degrees," Henry said. "Until they see an iceberg, they're not going to do it."

Japan is facing a massive reconstruction program in the region devastated by the tsunami, an ongoing nuclear energy crisis and unaddressed problems in the economy.

Noda, a fiscal conservative, has pledged to raise taxes and would like to privatize state assets.

Last week, the credit rating agency Moody's downgraded Japan to an Aa3 rating from Aa2, blaming the country's huge deficit and frequent changes in administration that have prevented the government from implementing long-term economic policies.

The CIA World Factbook puts the government debt at more than 200% of the GDP.